Writing Formulas
and Naming
Ionic & Covalent
Compounds
©2005 Douglas Gilliland
Honors Physical Science @ Sarasota High
Sarasota, Florida
Chemical Compounds
An atom consist of a positively charged nucleus (protons
and neutrons) and orbiting electrons.
Atoms of elements are not chemically stable until they have 8
electrons (octet rule).
Atoms gain, lose or share electrons with other atoms to be
come chemically stable ( have 8 valence electrons ).
Six valence
electrons.
Eight valence
Not chemically
stable.
-2
8+
Oxygen Atom
electrons.
8+
stable.
Oxide Ion
Chemically
Types of Compounds
I o n i c - occurs when a metal loses all
its valence electrons to a nonmetal. Metal
becomes a cation, nonmetal an anion.
C o v a l e n t - two nonmetals share
electrons. Neither loses or gains
electrons - they share electrons. Neither
atom becomes and ion.
Ionic
Compounds
Crystalline Lattice
Burning Magnesium
2Mg + O2 → 2MgO
20+
+
-2
+2
8+
Magnesium Atom + Oxygen Atom
20+
➝
+
Magnesium Ion
+
8+
Oxide Ion
Reaction between sodium + chlorine
2Na + Cl2 → 2NaCl
11+
+
17+
Sodium Atom
+
Chlorine Atom
11+
➝
+
Sodium Ion
-
+
17+
+
Chloride Ion
Ionic Compounds
Includes a metal and a nonmetal.
Metal loses e- and becomes a cation (+).
Nonmetal gains e- and becomes an anion(-).
Metal comes first followed by nonmetal.
Change the name of the nonmetal to -ide.
Examples: nitride, sulfide, fluoride, oxide,
bromide, iodide, chloride, telluride, phosphide.
Valence electrons are electrons in the
outer energy level of an atom.
Metals have between 1-3 valence electrons.
Nonmetals have between 5-7 valence electrons.
Lose e-
Cations (+ ions)
Gain eAnions
(- ions)
The valence of an elements is the charge it takes
when it loses or gains electrons.
Metal atoms lose electrons and become + ions (cations)
Nonmetals gain electrons and become - ions (anions)
+1
+2
Transition Metals
Multiple valences
+3 -4 -3 -2 -1
0
The 5 Steps for writing an
ionic compound formula:
Write the symbols of the two elements.
Write the valence of each as superscripts.
Drop the positive and negative signs.
Crisscross the superscripts so they become
subscripts.
Reduce when possible.
Formula for boron oxide
1. Write the symbols of the two elements.
B O
Formula for boron oxide
2. Write the valence for each element.
+3
-2
B O
Formula for boron oxide
3. Drop the positive & negative sign.
3
B O
2
Formula for boron oxide
4. Crisscross the superscripts so they
become subscripts.
3
B O
2
Formula for boron oxide
4. Crisscross the superscripts so they
become subscripts.
B O
2
3
Formula for boron oxide
5. Reduce subscripts when possible.
(not possible here)
B O
2
3
Examples of Reduction
Ca 2S 2 → CaS
Ge 2 O4 → Ge O2
Be2Te 3 → can’t reduce
Al 3 N 3 → AlN
Sn 2 O4 → SnO 2
Most Transition elements have 2 valences.
Roman numerals are used in the name
to show the valence on the ion.
9I
:R
<
Period 4 Transition Metals
)X
3T
,K
)U
4O
)[
@T
Examples:
Mn Manganese(IV) Mn
+4
Fe
+2
Iron(II)
Fe
+3
Cu Copper(I) Cu
+1
+6
+2
Manganese(VI)
Iron(III)
Copper(II)
Examples of Transition Metals
+2
Iron(II) Fe
+3
Iron(III) Fe
+1
Copper(I) Cu
+2
Copper(II)Cu
+2
Manganese(II) Mn
+4
Manganese(IV)Mn
Naming compounds with a transition metal.
formula
name
Fe2 O 3
Iron(III) oxide
ZnCl 2
Zinc(II) chloride
AgCl
Silver(I) chloride
Cu 3 P2
Copper(II) phosphide
PbS2
Lead(IV) sulfide
MnO2
Manganese(IV) oxide
Polyatomic Ions
-1
Polyatomic (many atom) ions are covalent
molecules with a charge. They behave as
if they were a single atom ion.
Polyatomic Ions
Nitrite
-2
SO2
-2
SO3
-2
CO3
Carbonate
Nitrate
-3
PO4
Phosphate
+1
NH4
-1
Ammonium
-1
NO2
OH
-1
NO3
Hydroxide
Sulfite
Sulfate
Treat polyatomic ions as you would any ion - crisscross to
determine the formula. The only difference is that when you
have more than one of a specific polyatomic ion in a formula
you must encase it in parenthesis.
Writing Formulas
Cation
Anion
Ca
-1
NO3
Mg +2
-3
PO4
+2
Na
+1
+2
Ba
OH
-1
-2
SO4
Compound
Ca(NO3)2
Mg 3(PO4)2
NaOH
Ba SO4
As in all ionic compounds you must reduce subscripts, but you
cannot change the formula of the polyatomic ion. You can only
reduce subscripts outside the parenthesis.
Compounds with Polyatomic ions
Carbonate
-2
CO 3
Sodium carbonate Na 2CO3
Calcium carbonate Ca CO 3
Phosphate
Aluminum carbonate Al 2(CO 3)3
-3
PO4
Sodium phosphate Na 3PO4
Calcium phosphate Ca3(PO3 )2
Aluminum phosphate AlPO 3
Covalent
Compounds
Water
Molecule
Hydrogen Oxygen
Atom
Atom
Hydrogen
Atom
Covalent Compounds
Two nonmetals share electrons so both
have 8 valence electrons. Exception: H
Neither takes on a charge - no valence.
Must use prefixes in the name.
Name tells you the formula. Example:
N2O4 is dinitrogen tetraoxide.
You cannot reduce the formulas!!!
Reaction between hydrogen + oxygen
2H2 + O2 → 2H2O
1+
1+
+
2 Hydrogen Atoms
8+
Oxygen Atom
1+
8+
Water Molecule 1+
The Space Shuttle
Oxygen
Hydrogen
Water Vapor
Exothermic
Reaction
2H2 + O2 → 2H2O
Covalent Prefixes
Mon - 1
Tetra - 4
Di - 2
Pent - 5
Tri - 3
Hex - 6
A prefix tells you the number of
atoms of that element
in the compound.
Examples of Covalent
Compounds
N2O3
Carbon tetrahydride CH 4
Phosphorus pentoxide PO 5
disulfur trifluoride
S2F3
Dinitrogen trioxide
Ionic and Covalent Structure
Ionic compounds
form a crystalline
lattice - a repeating
pattern of ions.
+3
B ions
N-3 ions
Boron nitride
Water
H atoms
O atom
Covalent compounds
form individual
molecules that are
not connected to
each other.
Naming Binary Compounds
Does the compound have a metal?
Yes
No
Covalent
Ionic
(Metal cation + Nonmetal anion)
Place metal first followed by nonmetal ending in -ide
(Two Nonmetals)
Contain a Transition Metal? Place the nonmetal furthest to the
left on the periodic table first, then
No
Yes
the other nonmetal ending in -ide.
Use Roman
Do not use Use prefixes to tell the number of
Numerals to tell
atoms in the compound
Roman
the valence of
mon(o)-1, di-2, tri-3, tetr(a)-4,
Numerals
the metal.
pent(a)-5, hex-6
Examples:
Examples:
iron(III) oxide
copper(II) chloride
manganese(IV) oxide
silver(I) chloride
sodium chloride
magnesium nitride
aluminum fluoride
beryllium oxide
Examples:
dinitrogen trioxide, nitrogen trichloride,
phosphorus pentoxide, sulfur dioxide
carbon tetrachloride, dihydrogen oxide
Summing up: Ionic
Ionic bonding occurs between a metal and
a nonmetal. Metals lose all their valence eand become cations. Nonmetals gain
enough e- to fill their valence level and
become anions.
Always crisscross valences and reduce to
determine the formulas of ionic compounds
Do not use prefixes in the names.
Ions form a crystalline lattice.
Summing up: Covalent
Covalent bonding occurs when two nonmetals
share electrons to fill their valence energy
level.
Never use valence to determine the formula there isn’t any valence. Since the two atoms
share electrons, they do not take on a
charge.
Always use prefixes in the names.
Atoms combine to form individual molecules.